Since 1987, when low operating voltages have been demonstrated by Tang et al. (C. W. Tang et al. Appl. Phys. Lett. 51 (12) 913 (1987)), organic light-emitting diodes have been promising candidates for the realization of large-area displays. They consist of a sequence of thin (typically 1 nm to 1 μm) layers of organic materials, which can be deposited, for example, by thermal vacuum evaporation or solution processing, followed by formation of the electrical contacts through metallic layers. Organic electrical devices offer a great variety of electronic or optoelectronic components, such as diodes, light-emitting diodes, photodiodes and thin film transistors (TFT), which, in terms of properties, compete with established components based on inorganic materials.
In the case of organic light-emitting diodes (OLEDs), light is produced and emitted by the light-emitting diode through the injection of charge carriers (electrons from one side, holes from another side) from the contacts into adjacent organic layers as a result of an externally applied voltage, subsequent formation of excitons (electron-hole pairs) in an active zone, and radiative recombination of these excitons.
The advantage of such organic components over conventional inorganic components (based on inorganic semiconductors such as silicon or gallium arsenide) is the option to produce large-area elements, e.g. large display elements (visual displays, screens) or lamps (for lighting applications). Organic materials, compared to inorganic materials, are relatively inexpensive (less expenditure of material and energy). Moreover, these materials, because of their low processing temperature compared to inorganic materials, can be deposited on flexible substrates, thereby opening up a whole series of new applications in display and illuminating engineering.
The basic construction of such a component includes an arrangement of one or more of the following layers: Carrier substrate; hole-injecting (positive contact) base electrode which is usually transparent; hole-injecting layer (HIL); hole-transporting layer (HTL); light-emitting layer (EL); electron-transporting layer (ETL); electron-injecting layer (EIL); electron-injecting (negative contact) cover electrode, usually a metal with low work function; and encapsulation, to exclude ambient influences.
While the foregoing represents the most typical case, often several layers may be (with the exception of HTL and ETL) omitted, or else one layer may combine several properties.
The use of doped charge-carrier transport layers (p-doping of the HTL by admixture of acceptor-like molecules, n-doping of the ETL by admixture of donor-like molecules) is described in document U.S. Pat. No. 5,093,698. Doping in this sense means that the admixture of doping substances into the layer increases the equilibrium charge-carrier concentration in this layer, compared to the pure layers of one of the two substances concerned, which results in improved electrical conductivity and better charge-carrier injection from the adjacent contact layers into this mixed layer. The transport of charge carriers still takes place on the matrix molecules. According to U.S. Pat. No. 5,093,698, the doped layers are used as injection layers at the interface to the contact materials, the light-emitting layer being found in between (or, when only one doped layer is used, next to the other contact). Equilibrium charge-carrier density, increased by doping, and associated band bending, facilitate charge-carrier injection. The energy levels of the organic layers (HOMO=highest occupied molecular orbital or highest energetic valence band energy; LUMO=lowest unoccupied molecular orbital or lowest energetic conduction band energy), according to U.S. Pat. No. 5,093,698, should be obtained so that electrons in the ETL as well as holes in the HTL can be injected into the EL (emitting layer) without further barriers, which requires very high ionization energy of the HTL material and very low electron affinity of the ETL material.
With respect to active OLED displays, so-called crosstalk between pixels of the display has been a major problem. Pixel or colour crosstalk refers to photons of one colour generated by a pixel falsely mixing with photons of another colour scattered from a close pixel. For example, documents GB 2 492 400 A and WO 2002/015292 A2 provide measures for reducing colour crosstalk in OLED devices. In addition, or as an alternative aspect, electrical crosstalk may occur. In this case, for example, a driving current applied to one of the pixels may cause light emission from another pixel close to the pixel for which the driving current is provided. Both will have a negative impact on the performance of the display device. (see Yamazaki et al., A. (2013), 33.2: Spatial Resolution Characteristics of Organic Light-emitting Diode Displays: A comparative Analysis of MTF for Handheld and Workstation Formats. SID Symposium Digest of Technical Papers, 44: 419-422. doi: 10.1002/j.2168-0159.2013.tb06236.x).
In a typical commercial active matrix OLED display, electrical pixel cross talk may be caused by the application of redox p-doping in a hole transport layer (HTL) which is shared by more OLED pixels (in the sense that the shared HTL is electrically connected to anodes of a plurality of pixels present in the display). The use of redox p-dopants which increase charge carrier density by creation of new charge carriers (holes) by transfer of an electron from a molecule of the doped matrix to a dopant molecule is beneficial for low-operating voltage, high operational stability and high production yield. On the other hand, redox p-doping increases electrical conductivity of hole transport layers from less than 10−8 S/cm without p-dopant, usually from less than 10−10 S/cm, to more than10−6 S/cm (typically, with concentrations of the p-dopant in the range between 1 and 5 wt. %). Therefore, redox-doped HTL is usually responsible for any electrical pixel cross talk in active matrix displays comprising a HTL shared by plurality of pixels. The ETL, if n-doped with redox n-dopants, might also show similarly high conductivity as the redox-doped HTL, however, due to display layout with a common cathode, the ETL does not cause electrical pixel cross talk.